1. Field of the Invention.
The invention relates generally to devices used to remove condensate (liquid created from the condensation of a gas or vapor) from steam lines and/or equipment or volatilized liquid lines and/or equipment, and more particularly to a steam trap system that has electronic sensors for monitoring the condensate level within the steam trap, an electronic control system for both monitoring steam trap performance as well as adjusting the opening of a condensate discharge valve to maintain condensate level within a control band, a flashing utility which permits reuse of high temperature condensate, and an electronic temperature/pressure sensing device that will constantly monitor temperature and pressure variables in the individual condensate lines to verify the integrity of the individual condensate lines that are going into a single manifold of a steam trap.
2. Description of the Related Art.
In a steam system, a boiler or steam generating unit is supplied feedwater (water which has cycled through the steam system or makeup/city water) which is heated to the saturated liquid state, vaporized to the saturated vapor state (steam), and then superheated. The steam produced may be used to transfer heat to a process. The steam leaves the boiler via the main steam line and enters the main steam header. From the main header, piping directs the steam to the steam heating equipment. As the steam performs its work in manufacturing processes, turbines, building heat, etc. (collectively, Process), the steam transfers its heat. As the steam releases this heat, it is cooled and reverts back to a liquid phase called condensate.
If condensate backs up in the steam system, much inefficiency will occur. The heat transfer rate to the Process is greatly reduced. Also, condensate backed up inside of the steam system piping cools the tubes that carry the steam to the Process. When this sub-cooled condensate is suddenly replaced by hot steam, the expansion and contraction of the tubes stress the tube joints. Constantly repeating this cycle causes premature failure. Finally, water hammer may result. Water hammer may occur where an accumulation of condensate (water) is trapped in a portion of horizontal steam piping. The velocity of the steam flowing over the condensate causes ripples in the water. Turbulence builds up until the water forms a solid mass, or slug filling the pipe. This slug of condensate can travel at the speed of the steam and will strike the first elbow in its path with a force comparable to a hammer blow. This force may be strong enough to break the pipe.
To solve these problems, steam traps have been long used in steam piping and in steam operated equipment to prevent the build-up of condensate formed by the condensation of steam in lines from the boiler. The goal of these steam traps is to drain the condensate as well as discharge air and non-condensable gases without permitting the steam to escape. If steam is allowed to escape, heat that should have been transferred to the Process will be lost. Steam traps are located after the main steam header throughout the system. Multiple pipes conducting steam to the Process may connect to a single manifold which conducts condensate to the steam trap. The condensate passes through the condensate return line and is collected and directed back to the boiler to repeat the water to steam process. Removing the condensate prevents damage to steam lines, steam turbines, steam pistons and other equipment that is operated and/or powered by the energy contained within the steam. Additionally, condensate removal, in some cases, may prevent water damage to the goods being manufactured.
Steam traps commonly used fall into four categories: mechanical steam traps, thermodynamic steam traps, thermostatic steam traps, and electronic steam traps.
Mechanical steam traps work on the principal of differentiating between the density of steam and condensate. The inverted bucket is a type of mechanical steam trap. In the mechanical steam traps, a valve opens and closes depending on the level of condensate in the steam trap bowl. For instance, in the inverted bucket steam trap, condensate enters the steam trap chamber from the bottom. As the level of condensate rises, the inverted bucket rises until it actuates a mechanical valve which allows the condensate to be blown by steam pressure into the condensate recovery lines. In the presence of steam only, the inverted bucket/float does not become buoyant, but sits securely over the orifice to close the steam trap.
The thermostatic steam traps operate by sensing the temperature of the condensate. As steam condenses, the condensate so formed is at steam temperature, but as it flows to the steam trap, it loses temperature. When the temperature has dropped to a specified value below the steam temperature, the thermostatic steam trap will open to release the condensate. For instance, this type of steam trap might have a bellows filled with a fluid that boils at steam temperature. As the fluid boils vapors expand the bellows to push the valve closed. When the temperature drops below steam temperature, the bellows contract to open the valve and discharge condensate. The bimetallic steam trap is an example of a thermostatic steam trap.
Thermodynamic steam traps operate on the principal of the difference between the flow of steam over a surface compared to the flow of condensate. Steam flowing over a surface creates a low-pressure area thus these steam traps are designed to open when the condensation of steam within the steam trap causes a change in pressure.
Electronic steam traps have also been developed to remove condensate from steam lines. Examples of electronic steam traps include Green, Rasmussen, Koch, Bridges, and Lin.
U.S. Pat. No. 3,905,385, Sep. 16, 1975 (Green) illustrates what appears to be the first use of an electronic sensor in the steam trap. Green shows the use of a condensate sensor connected directly to a condensate discharge valve. When the condensate level reaches the level of the discharge sensor, a circuit will be completed which will actuate the solenoid and open the condensate discharge valve.
U.S. Pat. No. 5,469,879, Nov. 28, 1995 (Rasmussen). This steam trap removes condensate on demand similar to a mechanical steam trap with the difference being that a electrical sensing probe extends into the condensate collection chamber and senses the high and low levels of condensate. When the condensate reaches the sensing probe an electric current causes a valve to open and the condensate is purged from the steam trap chamber. Once the condensate level falls below a preset level the valve will close until condensate again accumulates in the steam trap. This system also discloses an alarm circuit, which will indicate lack of valve opening or unusually high or low levels of condensate within the steam trap chamber.
A third example is described in U.S. Pat. No. 4, 974,626, Dec. 4, 1990 (Koch). Koch discloses an electronic-controlled steam trap that also uses an electrical sensor to control a discharge valve. In addition to Rasmussen's high and low level sensors, Koch also has a timedelay circuit. The upper sensor or the high-level sensor will actuate the discharge valve. The valve will stay open until a specified time-delay after the condensate level has reached the low-level sensor. Thus, the steam trap will drain the level of condensate past the low-level sensor. The purpose of the time delay is to insure that steam trap drains completely and that the flashing of condensate within the steam trap chamber during drainage does not prevent complete drainage of the steam trap.
U.S. Pat. No. 4,505,427, Mar. 19, 1985 (Bridges) provides a fourth example of an electronic steam trap. This steam trap is designed to prevent the steam trap from becoming locked by a bubble of steam around the electronic probe. The circuit includes a timing circuit that is reset each time the valve opens. If the condensate discharge valve does not open after a predetermined time then the valve will open even if the sensor does not detect condensate. This periodic cycling of the discharge valve prevents steam locking of the valve and a build-up of condensate within the system that could damage equipment.
U.S. Pat. No. 4,308, 889, Jan. 5, 1982 (Lin) illustrates a fifth electronic steam trap similar to those discussed above. The difference between Lin and the other steam traps discussed is that this steam trap adds an orifice between the probe and the discharge valve to limit the flashing of the condensate to steam when the discharge valve opens. This reduces the possibility for steam locking the probe and of receiving a false indication that the steam trap has been drained.
The general principle behind all of these different types of steam traps is the desire to limit the loss of steam and/or pressure (energy) while maintaining the steam lines and equipment free from condensate. However, the design of these steam traps causes the steam trap to cycle quite often leading to steam trap damage and failure. When a steam trap fails to properly drain, condensate remains in the steam lines and equipment and causes damage. If the steam trap falls to close, live steam enters the steam trap and there is a loss of steam pressure and heat/energy. Also, valve damage is particularly likely when the valve is in the process of closing in the presence of a mixture of condensate and steam traveling at high velocities. The high velocity water drops will result in the valve seat and valve disc being steam cut and eroded. This damage to the steam valve will ultimately require either the replacement or repair of the valve disc and the valve seat. This results in a reduction in the operating efficiency for the steam plant.
The constant cycling of these steam traps causes the steam pressure on the steam plant equipment to also cycle. This cycling pressure is suspected to reduce the life expectancy of siphon pipes used to remove condensate from some steam equipment. Rotary joint siphon tubes employed to remove condensate from dryer drums used in paper and box plants appear to be the most susceptible to damage.
Another drawback to these steam traps is the phenomenon of condensate flashing. Condensate flashing occurs when there is a decrease in pressure in very hot condensate. The steam system produces superheated steam. This is possible because the saturation temperature or boiling point of water is a function of pressure and this temperature rises when pressure increases. When water under pressure is heated its saturation pressure rises above 212 degrees F in the boiler. To illustrate, one pound of water at 70 degrees will remain water at 0 pounds per square inch (PSI). But 1 pound of water may be heated to 338 degrees F at 100 PSI before it will change from water to steam. This allows the steam to be superheated. Therefore, when very hot condensate is released from the steam trap into a lower pressure environment, the saturation temperature will lower correspondingly. This may cause the condensate to "flash" into steam which is then dissipated throughout the system. Besides the inefficiency of not utilizing the energy released by this phenomenon, flashing may cause a second form of water hammer in the steam system. This type of water hammer is called cavitation. Cavitation is caused by a steam bubble forming or being pushed into an area completely filled with water. When high temperature condensate flashes upon leaving the steam trap, such steam bubbles may form. As these steam bubbles are pushed into colder condensate in the return piping, the trapped steam bubbles will lose their latent heat and the bubble will implode. As the bubble implodes, the wall of water comes back together and the force created can be severe. This condition can damage piping and potentially damage the steam trap itself. The most common method of dealing with this phenomenon is to install a flash tank where the condensate is being discharged into the return condensate line. This holding tank allows the condensate to flash without releasing air bubbles into the condensate return line. The steam produced in this flash tank merely dissipates into the steam system. Therefore these steam traps do not utilize the energy contained in the flash steam. The steam traps described above, merely return the condensate to the condensate return line where the continually cooling condensate is directed back to the boiler to repeat the water to steam process. En route to the boiler, condensate loses even more heat which simply dissipates into the system.
The disadvantage to this loss of energy may be illustrated by describing the use of a steam system to power a corrugated cardboard plant. Corrugated paperboard is manufactured at high production rates on corrugator machines that are well known in the paper industry. The corrugator machine unwinds two continuous sheets of board from rolls; these sheets are softened/conditioned by steam at 135 PSI by a high-pressure condensate pump. The corrugator machine then presses flutes into the sheet of corrugated medium, applies glue to the tips of the flutes and then adds a third sheet of linerboard to form corrugated board. The combination of these sheets is called the web. To complete the formation of corrugated board, the adhesive is cooked/cured by passing the freshly glued web across a series of hot plates under pressure from above. The hot plates may be heated internally by steam to a temperature necessary to cure the adhesive. Pressure is provided by moving the web over the hot plates under a belt which rests upon the web and advances with the web at the same speed. Weight rollers on top of the belt provide additional pressure to hold the web together and maintain them flat against the hot plates to enhance heat transfer from the hot plates to the web for the curing process. As the heat acts upon the adhesive, it also drives moisture out of the web so that the finished corrugated paperboard will exit the heating section in a stiffened, substantially flat condition. The web then passes immediately through a cooling section to reduce its temperature prior to cutting the board.
The process of curing adhesive in the web may cause warping, cracking, or crushing of the board due to a combination of the high temperature of the hot plates, uneven moisture contained in the web, pressure provided by the weighted belt above the web, and the sudden cool-down process. Thus, it is necessary to reduce the temperature of the hotplate section. In order to reduce the temperature of the hotplates, the steam must be reduced to a lower saturated pressure and temperature. One way in which this is accomplished is by using a pressure reducing valve and a desuperheater station which sprays condensate into the reduced pressure steam to bring it back to a saturated condition. U.S. Pat. 5,656,124 suggests another means of alleviating this problem by providing an improved hot plate section utilizing a steam manifold having multiple valves allowing the operator to vary the pressure and therefore the steam temperature applied across the moving web. By varying the temperature of the steam, an optimum temperature for heating the web may be achieved that will not result in warping, cracking, or crushing the web. The need for varying steam temperatures in this industry indicates a potential use for heat contained in return condensate if such heat could be harnessed and re-introduced to the system.
Another drawback to the use of steam traps is that air and other noncondensible gases leak into the steam trap chamber when the steam is shut off and the steam system cools down. Steam is contaminated with a small amount of carbon dioxide gas when it leaves the boiler and air is drawn into steam heating equipment through rotary joints and valve packing glands. When steam inside the steam system condenses into water and is drained out through the steam trap, it creates a vacuum inside the heating equipment which is filled with air and other noncondensable ad gases. Neither air nor carbon dioxide condense, therefore, they must be pushed out of the steam system by the steam when the heating equipment is put in service again. If the air and noncondensable gases are not forced out of the system, a good heat exchange cannot take place. For instance, in a cascade type steam system, warm-up steam from the main steam header is admitted to the first steam user unit, the air in this unit must be forced out to the second unit. Once the first unit is warmed up, the warm-up steam is admitted to the second unit which eventually forces the air out of this unit into a third unit. This continues until all of the air is pushed back into the boiler tank which ultimately results in more residue in the boiler tank that must be removed during a system blow-down. Furthermore, if air/noncondensable gases are trapped within the steam trap, erroneous readings may result. Prior art traps accomplish this air purge via a very small orifice in the top of the steam trap. The slow warm-up procedure necessary in steam systems, to prevent shocking the metal pipes, and the fact that steam pressure in the steam system must increase enough to push the air out of the orifice in the steam traps and back to the boiler room results in a less efficient system.
Therefore, there is a need for a steam trap that limits the cycling of the steam trap and thereby extends the steam trap life. There is further need for a steam trap that will control condensate flash to prevent water hammer from damaging the steam system while utilizing the heat contained in condensate to provide further power to the Process. There is a need for a steam trap that can efficiently discharge air and other noncondensable gases. Finally, there is also a need to monitor a steam system in which multiple condensate lines feed into a steam trap. If one of the lines is damaged, the resulting loss of pressure and loss of heat will render the entire system more inefficient. Currently, there is no way to identify which of the several lines is adversely impacting the system as a whole.